The present invention relates to an apparatus for processing video signals representing an X-ray image obtained from an X-ray television camera, and more particularly to an apparatus for image information processing suitable for noise reduction.
An X-ray fluoroscopic system provided with an X-ray television camera has the disadvantages that the X-ray dosage can be increased only within a certain limit because of the necessity to restrain the X-ray dosage to which a subject is exposed. The result is that a fluoroscopic picture obtained from such a system inevitably contains noise.
A conventional way of noise reduction is to allow objectionable lag to occur by allowing video signals to pass through a recursive filter. The trouble is that, when an image in motion is allowed to pass through the recursive filter in the same manner as an image at a standstill, the leg shows itself in the form of a vague out-of-focus picture. In order to eliminate such vagueness, it is common to incorporate a motion detector, which allows the recursive filter to have a stronger effect on an object at a standstill and a weaker effect on an object in motion.
FIG. 2 shows the above-described construction of a recursive filter, into which a frame memory 6 for storing a frame of image is incorporated. The last frame of image is stored therein in the form of picture elements. Let it be supposed that one of them has a value B, which is multiplied by K in a multiplier 8. The corresponding one of picture elements, of which the present frame of image consists, has a value A, which is multiplied by (1-K) in a multiplier 2. The outputs taken from the multipliers 8 and 2 are fed to an adder 3, in which the following operation is performed: EQU A (1-K)+BK
The result of this operation is fed to the frame memory 6 and allowed to take the place of data hitherto stored therein. In the motion detector 11, each picture element of the present frame of image is compared with the corresponding picture element of the image stored in the frame memory 6. If the value of the picture element is found to have a change, it indicates that the object has moved. The value K is made small in this portion so as to allow the recursive filter to have a weaker effect.
Consequently the objectionable lag, as well as the out-of-focus picture resulting thereform, hardly occurs in the image of the object in motion.
However, the fact that the above-described conventional construction allows the recursive filter to have a weaker effect on the object in motion is tantamount to the fact that the image of the object in motion is displayed without undergoing any processing for noise reduction. Thus the image of the object in motion contains noise.